The present invention relates generally to alarm systems and more particularly to an annunciator which activates an alarm signal upon opening of a door.
There are many instances in which there is a requirement for an alarm to sound automatically upon the opening of a door. For example, in retail establishments such an alarm is needed to alert a salesperson if someone enters. A simple alarm system which sounds upon the opening of a door includes a switch which is engaged by the door whenever the door is fully shut in its frame. As soon as the door begins to open it disengages from the switch and the switch contact is thereupon activated to sound the alarm. A disadvantage of such a system is that the alarm sounds continuously whenever the door is not fully shut, and if the door remains open for more than a few seconds the continuously-sounding alarm becomes highly annoying.
To eliminate the annoyance of a continuously-sounding alarm, it has been proposed to utilize a switch which is momentarily activated by relative movement between the door and the switch. Thus, as the door is being opened it moves past the switch and momentarily sounds the alarm. However, as the door is being closed it again moves past the switch and again sounds the alarm, and this second sounding of the alarm has also been found to be annoying.
This problem could be overcome by a switch mechanism which responds to movement of the door in only one direction, and such mechanisms have been proposed. However, these mechanisms tend to be relatively complex. They are also subject to damage because they frequently project out from the door frame and get in the way. Moreover, two such mechanisms are generally required if the door is of the kind that can open either in or out.
It will be apparent from the foregoing that there is a need for a simple, economical door annunciator system which sounds an alarm briefly upon opening of a door but not upon a subsequent closing of the door.